The invention relates to loudspeakers and more particularly to resonant panel loudspeakers, e.g. of the kind described in International patent application WO97/09842, i.e. so-called distributed mode loudspeakers.
In the past there has been great difficulty in providing a loudspeaker covering the mid and high frequency audio range with high quality. Directivity varies greatly in this range and extension to the highest frequencies is very difficult for a mid driver alone. Typically two drivers are used with the expense and complication of a crossover network to divide the frequency range between them. The crossover frequency is generally around 3 kHz which is the most sensitive region in human hearing and which adds to the difficulty.
The concept of the present invention is to devise a mid and high frequency driver which replaces the two convention drivers previously used, which does not crossover in the critical region and which has consistent, desirably wide directivity throughout its working range.
Distributed mode loudspeakers can be designed to operate over some 8-Octaves of the audio frequency band, although this may not always be the best solution, for instance in hi-fi applications. It is envisaged that it might sometimes be appropriate for a distributed mode loudspeaker to be used in association with a subwoofer for low frequencies, crossing over, say, at around 100 to 200 Hz.
There is therefore a need for a loudspeaker or at least a loudspeaker drive unit that is not meant necessarily to work over 8-Octaves but perhaps over 6 to 7 Octaves of the audio band, which would allow a wide choice of material and various construction possibilities that would help optimise the loudspeaker over its operating range. An example of this is a hi-fi midrange/tweeter drive unit working under the distributed mode loudspeaker principle. This can bring significant benefits to a conventional boxed loudspeaker system by allowing the cross-over point to be designed away from the critical 3 kHz area down to 1 kHz range, typically 300-500 Hz as well as benefiting from the superior radiation properties associated with a distributed mode loudspeaker.
There is also an expanding market in multimedia and computer peripherals for high performance compact speakers and ever increasing demand for better sound and more compact construction for conventional televisions, monitors and flat panel televisions.
Thus there is a need for a structure that can be very compact and which can allow numerous features to be added for performance enhancement, application versatility and cost saving.
The present invention provides a cost-effective vehicle for all such applications and allows a manufacturer to optimise on tooling outlay and its production processes.
The basic concept revolves around a simple construction of the loudspeaker xe2x80x9cenginexe2x80x9d or drive unit which would allow easy production assembly and provide consistency.
According to the invention there is provided a mid/high frequency loudspeaker comprising a stiff lightweight resonant panel-form member, a housing to which the panel-form member is mounted, a suspension connected between the edges of the panel-form member and the housing and by means of which the panel edges are restrained against movement, the arrangement being such that the housing and the panel-form member together define a cavity at least partially enclosing a face of the panel, and an electrodynamic exciter for applying bending wave energy to the panel-form member to cause it to resonant to produce an acoustic output, the exciter comprising a magnet assembly rigidly fixed to the housing and defining an annular gap, and a voice coil and coil former assembly disposed in the annular gap and rigidly fixed to the panel-form member near to the geometric centre thereof, wherein only said panel suspension centres the voice coil and coil former assembly in the annular gap.
The bending stiffness of the panel-form member may be in the range 0.15 Nm to 24 Nm and is preferably in the range 2 Nm to 9 Nm.
The vibration exciter may be bonded to the panel-form member (herein after xe2x80x98panelxe2x80x99) and/or to the enclosure by way of injection moulding or by use of the adhesive. The vibration exciter voice coil may be bonded directly to a resonant panel during the injection moulding of the panel. Alternatively the voice coil of an exciter may be bonded into a pre-formed aperture moulded in a resonant panel during assembly. The need for a separate voice coil carrier is thus removed.
The panel may be co-moulded with the suspension. The suspension may be of a hard or semi-rigid foam plastics material. Alternatively, the panel may be fixed by adhesive means directly to the housing or enclosure, in which case the suspension is the adhesive connection between the panel and housing or enclosure.
The panel may be injection moulded as a monolith or using foaming techniques. The panel may be flat or curved and may vary in thickness or cross-section.
The enclosure may have embedded electrically conducting inserts for carrying electrical signals efficiently from connectors on the enclosure edge to the vibration exciter. The assembly and connectivity of the drive unit may thus be automated.
The mid/high frequency speaker may be clad in other mouldings and structures to suit the application, e.g. for aesthetic reasons. For example an appropriate trim will make it suitable for surface mounting onto a hi-fi speaker cabinet. The mid/high frequency speaker may be a drive unit or engine and the engine may be mounted onto other structures such as television cabinets.